You probably thought, because I`m using half the actual distance, the force must be 1/4th of the actual force, but that is not true.
You know the gravitational force and you know the angle. Since the ball is in equilibrium, the electrical force is just the gravitational force minus the tension.
Therefore, from your picture on the left: [itex]F=F_g\tan(18^{\circ})[/itex]